Elements and process for recording direct image neutron radiographs

ABSTRACT

An element is provided for recording a direct image neutron radiograph, thus eliminating the need for a transfer step (i.e. the use of a transfer screen). The element is capable of holding an electrostatic charge and comprises a first layer for absorbing neutrons and generating a current by dissipation of said electrostatic charge in proportion to the number of neutrons absorbed, and a second layer for conducting the current generated by the absorbed neutrons, said neutron absorbing layer comprising an insulative layer comprising neutron absorbing agents in a concentration of at least 1017 atoms per cubic centimeter. An element for enhancing the effect of the neutron beam by utilizing the secondary emanations of neutron absorbing materials is also disclosed along with a process for using said device.

Poignant, Jr. et a].

June 3, 1975 1 i ELEMENTS AND PROCESS FOR RECORDING DIRECT IMAGE NEUTRONRADIOGRAPHS [75] Inventors: Robert V. Poignant, Jr.; Edwin P.

Przybylowicz, both of Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: July 20, 1973 [21] Appl. No.: 381,106

{52] US. Cl. 250/315; 250/327; 250/390 (51] Int. Cl. G012 3/00 [58]Field of Search 250/315 A, 327, 390, 391, 250/392 156] References CitedUNITED STATES PATENTS 2,825,814 3/1958 Walkup 250/3l5 A 3,390,270 6/1968Treinen et all 250/390 Primary Examiner.lames W. Lawrence AssistantExaminer-Davis L. Willis Attorney, Agent, or Firm-R. P. Hilst [57]ABSTRACT An element is provided for recording a direct image neutronradiograph, thus eliminating the need for a transfer step (Le. the useof a transfer screen). The element is capable of holding anelectrostatic charge and comprises a first layer for absorbing neutronsand generating a current by dissipation of said electrostatic charge inproportion to the number of neutrons absorbed, and a second layer forconducting the current generated by the absorbed neutrons, said neutronabsorbing layer comprising an insulative layer comprising neutronabsorbing agents in a concentration of at least 10 atoms per cubiccentimeter. An element for enhancing the effect of the neutron beam byutilizing the secondary emanations of neutron absorbing materials isalso disclosed along with a process for using said device.

22 Claims, 3 Drawing Figures ELEMENTS AND PROCESS FOR RECORDING DIRECTIMAGE NEUTRON RADIOGRAPHS FIELD OF THE INVENTION This invention relatesto neutron radiography and. more particularly. to elements for makingdirect-print neutron radiographic recordings.

BACKGROUND OF THE INVENTION Neutron radiography. i.e.. the production ofphotographic images formed as a result of the differential attenuationof neutrons by an object. is in many respects similar to X-radiography.Both techniques provide an image of the inside of an object. The twotechniques complement each other. in that many materials opaque to oneare transparent to the other. The field of nondestructive testing offersa particularly valuable area for the use of neutron radiography. It isknown that x-rays interact with orbiting atomic electrons, and x-rayattenuation is therefore proportional to material density and atomicnumber. Neutrons, however. interact with the atomic nucleus. and theirattenuation is proportional to nuclear capture cross sections and thedensity of atoms per unit volume but independent of atomic number.Accordingly. neutron radiography permits examination of many materialcombinations that cannot be effectively differentiated by x-rays. Forexample. neutron radiography is capable of detecting the presence andlocation of light elements such as hydrogen. beryllium. lithium. andboron within a block of lead.

Neutrons are produced in three ways: from nuclear reactions induced inan accelerator. a radioisotope. or a nuclear reactor. In each caseneutrons are removed from an atom during a nuclear transmutationprocess. Neutrons are available in the enormous energy range of 1Oelectron volts. specifically. from 10 to [0" eV.

Accelerator is a general name given to devices that accelerate a beam ofcharged particles and direct them onto a target. An interaction takesplace between the bombarding particles and target atoms. and thisresults in the expulsion of other particles. With particularcombinations of incident particle and target material the ejectedparticles are neutrons. Typical of such a system is a device whichionizes the atoms of deuterium gas and uses a CockcrofLWalton voltagegenerator 100-400kV) to accelerate the ionized atom onto a tritiumtarget. The reaction releases an l4MeV neutron.

Radioisotopes are produced by bombarding nuclei with charged particlesin an accelerator or nuclear reactor. A nucleus becomes radioactive whenit changes from a stable unexcited state to an unstable excitedcondition. The extra energy imparted to the nucleus to change it to theunstable excited condition is eventually emitted in the form of gammarays or other particles as the nucleus decays back to its stableunexcitcd state. Unfortunately there are few radioisotopes which emitneutrons. Neutron production involving radioisotopes is generallyachieved by bombarding a target with gamma rays or alpha particlesemitted from the radioisotope. There are a few radioisotopes which decayby spontaneous fission to produce neutrons. but of these onlycalifornium-ZSZ has sufficient output to be considered.

The nuclear reactor is a device that produces fast and slow neutrons.gamma rays and charged particles. all in prolific quantities. Generallythose reactor sources which have a relatively high thermal neutronintensity appear most useful because great relative differences inneutron absorption cross-section exist for thermal neatrons. The nuclearreactor is the most intense source of neutrons and therefore has beenthe most widely used for neutron radiography.

Two forms of neutron radiography have been practiced using photographicfilm for recording purposes. They are transfer neutron radiography anddirect neu tron radiography. Transfer neutron radiography may also bepracticed with conventional electrographic elements. This process. whichwill be referred to herein as transfer neutron electroradiography ormore simply. the transfer process. is a combination of two knownprocesses. namely transfer neutron radiography and electrophotography.and consists of:

l. exposing a potentially radioactive metal foil (transfer screen") inan imagewise fashion to a neu tron source to induce artificialradioactivity in the new tron-exposed regions. which radioactivity isproportional to the intensity of the absorbed neutron flux;

2. removing (transfer step) the metal foil (which is now radioactive inan imagewise fashion) and placing it in close proximity to aconventional charged electrographic element such as a photoconductiveelement having a uniform surface charge. this operation being performedin a way not to expose the electrographic element prematurely to light.or a. B and 7 radiation;

3. allowing the radioactive emanations from the transfer screen tostrike the electrographic element for a period of time sufficient tocreate an imagewise discharge; and

4. removing the transfer screen and developing the radioactively-inducedelectrostatic image by any one of several electrographic developmenttechniques.

It would be advantageous if a direct neutron electroradiographic"process were available. This is because. among other reasons. fewersteps would be in volved with a direct neutron electroradiographicprocess than with the above-described "transfer process". Besides beingcumbersome. the transfer process suffers from additional problemsincluding possible image degradation, the half-life of the transferscreen. and disposal of the transfer screen. The principal cause of thedegradation is the small. but finite. separation which should bemaintained between the transfer screen and the recording element. Thisseparation is necessary to prevent another possible source ofdegradation in image quality. that arising from an electricalinteraction between the transfer screen and the recording ele ment.

SUMMARY OF THE INVENTION According to the invention an element isprovided for recording a direct image neutron radiograph. the elementbeing capable of holding an electrostatic charge and comprising means.preferably in the form of a first layer. for absorbing neutrons andgenerating a current by dissipation of the electrostatic charge inproportion to the number of neutrons absorbed. the neutron absorbingmeans comprising an insulative layer comprising neutron absorbing agentsin a concentration of at least 10" atoms per cubic centimeter.

The concentration of neutron absorbing agent (C) is evaluated by theformula ('=nl ,,,"N". where n is the number of neutron interacting atomsper molecule. l',,

is the molor volume. and N" is Avogadros number. A general criteria forchoosing neutron absorbing agents include the following considerations:l for a particular neutron energy range of the source being used. thetotal neutron cross-section per atom should be at least one barn; (2)for most applications. neutron interac tion should not producelong-lived radioisotopes; and (3) the ratio of neutron to gamma rayabsorption coefl'tcients should be one or larger.

There is further provided an element for recording a direct imageneutron radiograph. said element enhancing the effect of the neutronbeam by emanating secon' dary radiation simultaneously while absorbingneutrons, and capturing said secondary emanations to form an imagewiseelectrostatic charge pattern.

In the direct neutron electroradiographic process of the presentinvention, the elements to be exposed is directly exposed by virtue ofneutron induced interac tions within the electrographic medium of theelement. Thus the danger arising from handling radioactive transferscreens is eliminated. Many practical neutron sources, however, have ahigh gamma ray flux in addition to neutron flux. This gamma ray fluxcould deleteriously tend to expose certain of the electroradiographicelements of the invention in addition to the desired exposure effectedby the neutron flux. thereby producing a hazed image. Therefore, inaccord with certain preferred embodiments of the invention there areprovided novel electrographic elements which are relatively insensitiveto the gamma radiation associated with most practical nuetron sources.

lt is the object of the present invention to provide novel radiographicelements which can be used for direct neutron electroradiography.

It is another object of the present invention to provide novelelectrographic compositions which are sensitive to neutron radiation.

It is still another object of the present invention to provide novelneutron sensitive electrographic elements which may be convenientlyhandled. exposed, and developed in the present of light.

It is still a further object of the present invention to provide novelelectrographic elements which are relatively insensitive to the gammaradiation associated with most practical neutron sources.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of theinvention will become apparent to those skilled in the art uponconsideration of the accompanying disclosure and drawing. in which:

FlG. illustrates in a simplified form, the essential features of apreferred neutron radiographic element in accordance with the invention;

PK]. 2 is a side elevational view of a neutron radio graphic element inaccordance with a preferred em bodiment of the invention; and

FIG. 3 is an enlarged view of the neutron radio graphic element of FIG.2 and illustrates the mechanism of the process for certain preferredmaterials.

Referring to FIG. 1, a neutron recording element 11 in accordance withthis invention basically comprises two layers. via. a neutron absorbinglayer 18 and a con ductive layer 19. The conductive layer 19 may serveas mechanical support for the neutron absorbing layer 18 if themechanical properties of the latter are insufficient to provideself-support. Typically a test object 16 Ill is placed between theneutron recording device I l and a neutron source 12. The neutron source12 emits a beam of neutrons l4 which expose the test object to theneutron recording device.

The neutron absorbing layer 18 is comprised of neutron absorbing agents.Neutron absorbing agents found useful in the practice of the presentinvention generally have relatively large neutron absorptioncoefficients. and. in addition. ratios of neutron to gamma ray absorption coefficients equal to or greater than unity. Table I shows acomparison of the thermal neutron and gamma ray mass absorptioncoefficients of chemical elements suitable for use in thermal neutronapplications. Similar data may be obtained for epithermal neutronapplications. and the like.

TABLE I ELEMENTS SHOWING GOOD THERMAL NEUTRON ABSORPTION CHARACTERISTICSMass Absorption ('oefficient The concentration of neutron absorbingagent in the neutron absorbing layer is evaluated for a pure sub stancefrom the formula nV,,, N. where n is the number of neutron interactingatoms per molecule. V,,, is the molar volume, and N" is Avogadro'snumber. The concentration of said neutron interacting atoms should be atleast l0 atoms per cubic centimeter. It is preferred. however. that theconcentration be in the range of from about 10 to about l0 atoms percubic centimeter.

A general criteria for choosing neutron absorbing agents to becompounded into useful materials for the neutron absorbing layer shouldbe based on three principle considerations: l for the particular neutronenergy range of the source being used. the total neutron cross-sectionper atom should be at least one barn. and preferably more than tenbarns; (2) for most applications, neutron interaction should not producelonglived radioisotopes; and (3t in order to minimize the influence ofdirect gamma ray interaction the ratio of neutron to x-ray absorptioncoefficients should be one or larger. especially when the source emits amixture of neutrons and gamma rays. The element boron is a good exampleof an element which satisfies all three requirements. It has a thermalneutron cross-section of 3.770 barns. does not produce long-livedradioisotopcs upon neutron interaction. and has a l7(lto-l ratio ofthermal neutron absorption coefficient to the lZS Ke\' gamma rayabsorption coefficient.

The sensitivity of the neutron radiographic element is dependent on thehalf-life of the neutron absorbing agent used therein. The length ofhalf-life of the neutron absorbing agent is directly proportional to thesen sitivity of the element. Neutron absorbing agents which producedlonglived radioisotopes. i.e. isotopes with relatively long half-li\es.are not generally suitable for rcuseable neutron radiographic elements.For reuseable elements. therefore. it is essential that isotopes of theneutron absorbing agent have a relatively short half-life. i.e. thehalf-life is short enough to decay radioactivity before the nextexposure of the element. For these reusable elements, it is preferredthat the isotopes of the neutron absorbing agent possess half-lives ofless than one second.

The thickness ofthe neutron absorbing layer 18 used in theelectrographic neutron recording process is based upon several factors.It is desirable to have as much neutron interaction as possible. subjectto the constraint that image quality not suffer via excessive lateralcharge diffusion. The thickness is governed by the concentration ofneutron absorbers per cubic centimeter, the neutron cross-section of theabsorber. the dielectric breakdown (maximum electrical field) strength.and the value of the applied voltage.

The neutron absorbing layer may be comprised of any materials that meetthe above requirements. provided that the electrical resistivity of thelayer is sufficiently high so that an electrical charge is retained onthe surface of the film. Generally, a resistance of 10'" ohm-cm or moreis required for satisfactory electro graphic performance. In somespecial uses. however. the resistance of the neutron absorbing layer canbe as low as [O ohm-cm. Typically, these neutron absorbing layers can bel films of solid organic materials having a high hydrogen content, i.e.a high concentration of neutron interacting hydrogen atoms per unitvolume. (2) composites of binder with neutron absorbing pig ments, and(3) vacuum deposited films comprised of electrically insulating neutronabsorbing compounds.

Useful solid organic materials having a high hydrogen content can bedetermined by calculating the concentration of neutron interactinghydrogen atoms with the formula nV f N". An example of a high hydrogencontent material is solid methane, which has 6 X hydrogen atoms percubic centimeter.

Organometallic compounds are also useful as neutron absorbing agents.Particularly useful organometallic compounds are those containingisotopes of boron, especially "5, and lithium. especially Li. Thoseorganometallic compounds containing natural isotopes of cadmium,Samarium, europium, gadolinium. gold, indium, silver, rhodium. anddysprosium are also useful in thermal neutron applications. Specificexamples include tri B-naphthyl borate; tri a-naphthyl borane; mono.bis, and tritmethylamine) borane; gadolinium hexaantipyrine chloride;hexaantipyrene gadolinium (Ill) perchlorate; triphenylborine amine;copper triphenylphosphineborane having the formula [P(C.,-H ),-,]-,CuBHand tetraarylborates as described in Photog. Sci. Eng. 16, 300-312.1972.

Compounds. particularly the oxides. nitrides and chalcogenides ofneutron interacting elements can be formulated into films appropriatefor use as the neutron absorbing layer. Especially useful inorganicmaterials include. for example: B 03. BN. BP. BC. CdS. CdF- (M 0 andgadolinium gallium garnet.

Films of these neutron absorbing materials can be formulated in manyways. For example. the agents can be dispersed in a binder. Materialscan be utilized in the form of coated or sintered layers as well ashot-pressed sheets. Films can also be \acuum deposited on anelectrically conductive support layer.

Binders useful in the preparation of neutron absorbing layers containingneutron absorbing pigments can be either organic or inorganic material.Typical exam ples of organic binders include. for example: natural andsynthetic plastic resins. waxes. colloids, gels and the like includinggelatins. desirably photographicgrade gelatin, various polysaccharidesincluding dextran. dextrin, hydrophyllic cellulose ethers and esters.acylated starches. natural and synthetic waves including paraffin.beeswax. polyvinylacetals. polymers of acrylic and methacrylic estersand amines. hydrolyzed interpolymers of vinyl acetate and unsaturatedaddition polymerizable compounds such as maleic anhydride, acrylic andmethacrylic esters and styrene, vinyl acetate polymers and copolymersand their derivatives. in cluding completely and partially hydrolyzedproducts thereof. polyvinyl acetate, polyvinyl alcohol. polyethyleneoxide polymers, polyvinylpyrrolidene, polyvinyl acetals includingpolyvinyl acetaldehyde acetal. polyvinyl butyraldehyde acetal. polyvinylsodium-osulfoben- Zaldehyde acetal. polyvinyl formaldehyde acetal andnumerous other insulating. film-forming binder materials known in theelectrographic art.

As is well known in the art. in the preparation of smooth uniformcontinuous coatings of binder materials. small amounts of conventionalcoating aids may be employed therewith as viscosity controlling agents.leveling agents, dispersing agents, and the like.

An example of an inorganic binder which may be found particularly usefulconsists of the vitreous form of H 0 The amount of neutron absorbingagents incorporated as a pigment in the binder may range from (Ll to99.9 percent by weight. The particular amount depends on the neutronabsorbing characteristics of both the pigment and the binder. The binderitself may or may not have significant neutron absorbing capacity. Inthe above example B. .O is a good neutron absorber. As long as the filmor layer formed contains the required concentration of neutroninteracting atoms, it does not matter how they are proportioned betweenthe binder and the pigment.

An insulative neutron absorbing glass, e.g. CdS-CdO- B 0 can also beprepared and utilized as the neutron absorbing layer. Cadmium sulfideprecipitation provides two useful glass compositions which expressed interms of weight percent are as follows: 41% CdS- 48.8'7r CdO 47. V7:8203([0 ohm/cm) and 4.9% CdS- 46.6% CdO-48.7% B 0; (3 X l0 ohm/cm). Bothglasses are insensitive to visible light and are insulators (see. forexample. Mem. Sci. Eng... Okayama Univ. Vol. 6, pp. 47-52 (l97l Theconductive layer 19 can be a conductive coating on a suitable filmsupport ofa conductive material with useful mechanical properties. Asuitable support mate rial should possess useful mechanical propertiesand should not exhibit a significant absorption crosssection forneutrons when the result of the absorptions are induced radioactivespecie with long half-lives. Any conductive support may be used as longas this test is met. For example. aluminum sheet metal is a suitablesupport element which interacts little with thermal neutrons. When ahydrogen rich polymer is used as the neutron absorbing layer. noadditional support may be needed and the conductive layer may be coatedor vacuum deposited on one side of the neutron absorbing layer.

It should be noted that although a conductive layer is a preferredfeature of the neutron elcctroradio graphic element it is not anessential feature. A conductive layer is not used in a process where thetop surface of the electrographic element is charged to one polaritywhile simultaneously charging the bottom surface to the oppositepolarity. Upon exposure. the charge in the image area dissipates Usingthe neutron recording element described above a process of directneutron radiography" can be practiced because this invention provides anovel class ofelectrographic materials which exhibit neutroninducedelectrical conductivity. A process of direct neutron electroradiographycomprises:

1. uniformly corona charging the surface ofa neutron sensitiveelectrographic element.

2. placing the neutron sensitive elcctrographic element behind theobject to be examined and providing a neutron exposure sufficient todischarge the electro- '1 graphic element in those areas where theUbJCCK is transparent to neutrons; and

3. developing the neutron induced electrostatic images by any of theseveral electrographic development techniques.

Many procedures can be utilized to obtain an electrostatic chargepattcrn and to obtain a developed image. Early work is described inCarlson U.S. Pat. No. 2.297.69l. issued Oct. 6. [942. wherein a chargepattern is formed and developed on a sensitive element. The chargepattern can be transferred to a receiver prior to development as in thevarious TEST processes (Transfer of Electrostatic Image] described inWalkup U.S. Pat. No. 2.833.648. issued May 6. 1958. Walkup U.S. Pat. No.2.937.943. issued May 24. I960. Carlson et al. US. Pat. No. 2.982.647.issued May 2. l96l. Dreyfoos et al. US. Pat. No. 3.055.006. issued Sept.18. i962 and Walkup US. Pat. No. 2325.814. issued Mar. 4. 1958. Gridcontrolled corona charging methods also can be used as described inFrank British Pat. Nos. l.l49.90l. issued Aug. 20. 1969. l.l52.308.issued Sept. lU. I969 and 1.52309. issued Sept. l0, I969. It is alsopossible to use a simultaneous charging and exposing mode of operationas described in U.S. Pat. No. 3.598.579 by G. H. Robinson, issued Aug.l0. l9? 1 The neutron clectroradiographic elements of this inventionmight conceivably be utilized in any known electrographic processes inwhich exposure by neutron is possible.

Conventional electroradiographic elements. such as those containing Seor PhD. etc. which are intended for X-ray application are not suitablefor use in the process described above. since they usually have largegamma ray absorption coefficients. but poor neutron absorptionproperties. Table [I shows a comparison of the neutron and gamma raymass absorption coefficients of chemical elements commonly used inmateri' als for X-ray applications.

TABLE ll ELEMENTS SHOWTNG GOOD GAMMA-RAY ABSORPTION COEFFICIENTS MassAbsorption Coefficients Ratio TABLE ll-Continued ELI-.Ml-INIS SHOWINGGOOD UAMMARAY ABSORPTION (Ol-FFICIENTS Mass Absorption ('oel'licientsNote that the chemical elements commonly used for X-Ray applicationsgenerally have large gamma ray ab sorption coefficients. but low neutronabsorption coefficients and ratios of neutron to gamma ray absorptioncoefficients less than unity.

Standard electroradiographic elements could be utilized in the directprocess if used in conjunction with a conversion screen and a neutronbeam with a low gamma radiation content. The conversion screen consistsof a material which absorbs and reacts with ncutrons and in turnemanates a. [3 or 7' radiation in proportion to the intensity of neutronabsorption. The secondary radiation then forms the imagewise pattern onthe recording element. A problem is that the recording element is alsoexposed to the neutron source and would react with any gamma radiationwhich is associated with most practical neutron sources. This couldproduce high background intensity. It is also not possi ble to examineradioactive objects by the direct process using a standardelectroradiographic element.

It is thought that neutron recording elements as herein described areuseful for electrographic neutron radiography applications because theelectrical conductivity of the neutron absorbing layer 18 increases whenexposed to neutrons. This allows a charge on the surface of the neutronabsorbing layer to be conducted through the layer to the conductivelayer 19 in direct proportion to the number of neutrons absorbed duringexposure.

In a further embodiment of the invention. some of the neutroninteracting agents of Table l which emit or. B. or y rays upon neutronactivation may be rendered conductive. The rays. if captured by asuitable material. can enhance the effect of the neutron beam.Radiographic sensitive materials which may contain chemical elementslisted in Table II can be utilized to capture these secondaryemanations. If these materials become conductive when exposed to thesecondary emanations of the Table l materials. they will effectivelyenhance the neutron beam effects.

In principle. any ionizing radiation sensitive. insulative compound canbe used to capture the secondary emanations of the neutron absorbingcompounds by placing it in close proximity or in contact with theneutron sensitive agent. This can be accomplished in a number of ways.for example: l by homogeneously dispersing an ionizing radiationsensitive insulative compound which may contain a chemical element ofTable II in a binder along with a neutron-absorbing agent (a chemicalelement of Table l is suitable for thermal neutrons) (2) by depositing alayer comprised of an ionizing radiation sensitive insulative compound(which may contain a chemical element of Table II on top of a layercomprised of a neutron-absorbing agent containing. for example. chemicalelement of Table I (in this case the neutron-abosorbing layer need notbe insulating); and (3) by placing an elcctrographic element comprisedof an ionizating radiation sensitive layer in close proximity (separatedby a narrow air gap for example) to an element comprised of a neutronabsorbing layer containing. for example. a chemical element of Table Ias illustrated by the electrographic clement in FIG. 2.

Typically, an clectrographic element comprising an ionizing radiationsensitive insulative layer and a neu tron-absorbing layer can beproduced. for example. by coating lead oxide or amorphous selenium ontop of a boron containing support. The insulative chargeaccepting layeris relatively transparent to thermal neutrons. However. the boroncontaining support will absorb thermal neutrons and convert them intoionizing radiation. The ionizing radiation. in turn. alters theconductivity of the insulative charge-accepting layer. A charge patternis then produced on the insulator surface in accordance with variationsin the neutron flux. In the above examples the boron containing support"can consist ofa conductive boron or a boride layer sup ported by aconductive base (such as sheet aluminum). Lead oxide and amorphousselenium exhibit considerable gamma ray interaction. Therefore materialslike these would not be desirable in situations in which the neutronbeam is contaminated with high gamma flux.

In a preferred embodiment. a neutron absorbing organic formulation iscoated on top of a boron or gadolinium coated conductive support toenhance the neutron sensitivity of the electrographic element. A neutron absorbing device as described above may be (a) used alone or (b)further enhanced by capturing the secondary emanations of theneutron-absorbing com pounds by coating thereon a layer containing anionizing radiation sensitive insulative compound.

Referring to FIG. 2, a preferred embodiment of the invention is shown.FIG. 2 shows a side elevation view of a neutron sensitive device exposedto a neutron flux 21. The neutron sensitive device is comprised of fivelayers. A first layer 22 is an electrically conductive layer. A secondlayer 23 is an insulative charge accepting layer. A third layer 27 is aninsulative layer which exhibits increased electrical conductivity whenstruck by ionizing radiation. A fourth layer 24 is a neutron sensitivelayer comprising an element from Table I which will produce secondaryemanations when struck by the neutron flux 2]. A fifth layer 25 isanother electrically conductive layer. Connected across conductive layer22 and 25 is a high voltage source 26.

The neutron sensitive device illustrated in FIG. 2 can be used in apreferred process to record the image of test object exposed to aneutron flux. In this process. a voltage is placed across the neutronsensitive device of FIG. 2 by connecting the conductive layers 22 and 25to opposite terminals of a high voltage source 26. A test object isplaced between the neutron sensitive device and a neutron source andexposed to a neutron flux. The neutrons are captured by the neutronabsorbing layer 24 of the neutron sensitive device. When struck byneutrons. the neutron absorbing layer 24 produces secondary emanations.Some of this ionizing radiation is back scattered to layer 27. aninsulative layer which exhibits increased electrical conductivity whenstruck by the ionizing radiation. A charge is thus conducted to theinsulative layer 23 which holds the charge. The charge pattern on layer23 is an imagewise pattern of the test object being exposed. This chargepattern on layer 23 is then developed by any conventional electrographicdeveloping technique.

The conductive layers 22 and 25 can be any conductive material. Iflayers 23 and 24 are self-supporting then layers 22 and 25 need be onlya conductive film. Generally, however. layers 22 and 25 contributemechanical strength to the other layers. A typically useful conductivematerial is. for example. aluminum sheet.

The charge accepting layer 23 can comprise an insu lating material. theprinciple requirement being the capability of holding a charge patternon its surface until development. Typically useful materials are thoseinstilative. filmforming polymeric materials which are commonly used inthe electrophotographie art as binders for photoconductors. Materials ofthis type include styrene-butadiene copolymers; silicone resins;styrenealkyd resins; siliconealkyd resins; soya-alkyd resins; poly(vinylchloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrilecopolymers; poly( vinyl acetate); vinyl acetate-vinyl chloridecopolymers; poly(vi nyl acetals). such as poly(vinyl butyral);polyacrylic and methacrylic esters. such as poly(methyl methacrylate).poly(n-butyl methacrylate). poly( isobutyl metha crylate), etc;polystyrene; nitrated polystyrene; polymethylstyrene; isobutylenepolymers; polyesters, such as polyl ethylene-co-alkylenebis(alkyleneoxyaryl phenylenedicarboxylate]; phenolformaldehyde resins;ketone resins; poly-amides; polycarbonates; polythiocarbonates;poly[ethylene-co-isopropylidene-2.2-bis(ethyleneoxyphenylene)terephthalate]; copolymers of vinylhaloarylates and vinyl acetate such aspoly(vinyl-m-bromobenzoateco-vinyl acetate); etc.

The insulative layer which exhibits increased electrical conductivity 27can be an air gap. a gap filled with some other insulative gas with highabsorption for ionizing radiation. or some other suitably insulatingmaterial which exhibits increased electrical conductivity when struck byionizing radiation. Typical examples of suitable insulating materialswhich exhibit these characteristics are any layers comprising a chemicalelement of Table II in an insulative. film-forming polymeric binder.

The neutron absorbing layer 24 may. for example, comprise any chemicalelement of Table I which has a suitably large neutron-cross-section andwhich produces secondary emanations upon absorbing. for example, thermalneutrons. The neutron absorbing layer can be electrically conductive inthis embodiment.

FIG. 3 illustrates a particular embodiment ofthe neutron recordingdevice of FIG. 2 in which the neutron absorbing layer 24 comprises ahot-pressed boron sheet. The reaction between boron and a neutron isgiven by the following equation:

'11 "B Li 4 The conductive layers 22 and 25 can be. for example.aluminum sheet and the insulative layer 27 (which is rendered conductiveby secondary emanations) is an air gap for purposes of this particularembodiment. When a neutron strikes and interacts with a boron nucleus areaction takes place according to the above equation. The secondaryemanations (alpha particles in this instance) thus produced can travelforward in the direction ofthc neutron beam as indicated at point X orthey can travel back into the air gap in a reverse direction to theneutron beam as indicated at point X la order to form an electrostaticcharge image of the neutrons. a high voltage potential is applied to thecon ductive elements during exposure to neutrons. Ionizing radiationfrom the neutronboron interaction creates ionization of air within theair gap. A Townsend discharge may be supported. The net result istransport of charge to the insulating charge-accepting layer 23 adjacentto the neutron-struck areas, but not elsewhere.

One special advantage of this mode of operation is the relativeinsensitivity of the charge accepting layer 23 to effects of gammaionized air. particularly if the converter 24 is not a gamma absorber.(Ionized air is a consequence of the gamma radiation that oftenaccompanies thermal neutrons.) This means that the above process is lesssensitive to certain gamma ray effects. Another advantage of this modeof operation is the possible high electrical gain (charge transferredper incident neutron) resulting from the radiation induced electricaldischarge in the insulative gap 27. Another is most secondary emanationsare charged particles which are readily absorbed in small distances bysimple gas such as nitrogen, oxygen. etc.

The following examples will illustrate the neutron induced conductivityof the neutronabsorbing layers of the present invention.

EXAMPLE l A 4 mil thick poly(ethylene terephthalate) film support havinga 0.24 OD (optical density) evaporated nickel layer was placed (nickelside down) upon a vacuum platen and charged in a negative corona to 2 KVsurface potential. The charged film and platen were located in the pathof a 14 MeV neutron beam having a flux of 8 X l0 neutrons/seocm for twominutes. The charged and exposed element was toned in an electrographicliquid developer comprising a colorant and a cobalt naphthenate chargeagent dispersed in a mixture of cyclohexane and lsopar G (anisoparaffinic hydrocarbon from Humble Oil Co.) and compared to a control film which had received an identical treatment ex cept that noneutron exposure was utilized. The neutron exposed film had acquiredlower quantity of toner (OD 0.38) compared to the unexposed controlsample (OD 0.47) because of the neutron induced discharge of theelectrostatic charge.

EXAMPLE 2 Twelve grams of nitrogen-flushed gadolinium oxide in aplatinum boat was heated in a quartz combustion tube for 60 minutes in amuffle furnace under dry nitrogen at 400C. The sample was cooled undernitrogen to room temperature. Ten grams of the gadolinium oxide wasball-milled 24 hours in a l20 cc. glass jar contain ing 6.7 grams of aPliolite S5 toluene mixture composed of 30 weight percent Pliolite S-S(styrene butadiene copolymer purchased by Goodyear Rubber Co.) and 7.0grams of toluene. The mixture was ball milled for 24 hours and thencoated at 50 percent solids on nickel coated poly(ethyleneterephthalate) support at a wet thickness of 0.010 inch using a doctorblade.

In order to measure the neutron induced current of the element madeaccording to the above procedure. electrodes were connected to thegadolinium oxide film and a constant bias was applied. When thedisplacement current had decreased to a very small value 1.2 X l0 A) thefilm was subject to a 14 MeV neutron exposure. The current through thefilm rose by nearly two orders of magnitude (to 76 X 10 A). increasingas the neutron flux was increased to l X It)" neutronslcm lsec. Thecurrent decreased abruptly (to 12 X l(J"- A) when the neutron source wasshut off.

The above examples illustrate the change in conductivity ofneutron-absorbing layers of the present invention when said layers aresubjected to a neutron flux.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof. but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. An element for reducing a direct image neutron radiograph, saidelement being capable of holding an electrostatic charge and comprisingmeans for absorbing neutrons and generating a current by dissipation ofsaid electrostatic charge in proportion to the number of neutronsabsorbed, said neutron absorbing means comprising an insulative layercomprising a neutron absorbing agent in a concentration of at least 10atoms per cubic centimeter.

2. An element for recording a direct image neutron radiograph. saidelement being capable of holding an electrostatic charge and comprisingmeans for absorbing neutrons and generating a current by dissipation ofsaid electrostatic charge in proportion to the number of neutronsabsorbed. and means contiguous with said neutron absorbing means forconducting said current. said neutron absorbing means comprising aninsulative layer comprising a neutron absorbing agent in a concentrationof at least l0 atoms per cubic centimeter.

3. An element according to claim 2 wherein the neu tron absorbing meanscomprises an insulative layer having a resistivity of at least 10'"ohm-cm.

4. An element according to claim 2 wherein the neutron absorbing agcntshave a ratio of neutron absorption coefficient to X-ray absorptioncoefficient of at least unity and are present in a concentration rangefrom about 10 to about 10'' atoms per cubic centime ter.

5. An element for recording a direct image neutron radiograph. saidelement being capable of holding an electrostatic charge and comprisingmeans for absorbing neutrons and generating a current by dissipation ofsaid electrostatic charge in proportion to the number of neutronsabsorbed. and means contiguous with said neutron absorbing means forconducting said current. said neutron absorbing means comprising aninsulative layer comprising a neutron absorbing agent in a concentrationof at least [0 atoms per cubic centimeter. said neutron absorbing agenthaving a neutron crosssection per atom of at least one barn, and havinga ratio of neutron mass absorption coefficient to gamma ray absorptioncoefficient of at least unity.

6. An element according to claim 5 wherein the neutron absorbing meanscomprises an insulative layer having a resistivity of at least l0ohnrcm.

7. An element according to claim 5 wherein the neutron absorbing agentsare present in a concentration range from about l0 to 10 atoms per cubiccentimeter.

8. An element according to claim 5 wherein the neutron absorbing agentis an organic material having a high hydrogen content.

9. An element according to claim 5 wherein the neutron absorbing agentis an organometallic compound containing an isotope of an elementselected from the group consisting of boron, cadmium, d) sprosium.europium. gadolinium. gold. indium. lithium. rhodium. sa marium andsilver.

10. An element according to claim 9 wherein the neutron absorbing agentis an organometallie compound containing an isotope selected from thegroup consisting of boron, "B. and lithium. "Li.

11. An element according to claim 9 wherein the neutron absorbing agentis an organometallic compound selected from the group consisting of triB-naphthyl borate.

tri a-naphthyl borane.

methylamine borane,

bis(methylamine)borane.

tritmethylamine)borane,

gadoliniumhexaantipyrine chloride.

hexaantipyrene gadolinium (Ill) perchlorate.

triphenylborine amine, and

copper triphenylphosphineborane.

12. An element according to claim 9 wherein the organometallic compoundcomprises a tetraarylborate.

13. An element according to claim 5 wherein the neutron absorbing agentcomprises an element selected from the group consisting of berylium,boron, cadmium, carbon. chlorine. eupopium, gadolinium. hydrogen,lithium. nitrogen, oxygen and samurium.

[4. An element according to claim 13 wherein the neutron absorbing agentis selected from the group consisting of B BN, BP, BC. CdS, CdF Gd O andgadolinium gallium garnet.

15. An element according to claim 5 wherein the neutron absorbing agentis a composition comprising CdS, CdO and B 0 16. An element according toclaim wherein the neutron absorbing agent is a composition comprisingabout 4.l percent by weight CdS, about 48.8 percent by weight CdO, andabout 47.1 percent by weight B 0 17. An element according to claim 15wherein the neutron absorbing agent is a composition comprising about4.9 percent by weight CdS. about 46.4 percent by weight CdO, and about48.7 percent by weight B 0 18. An element for recording a direct imageneutron radiograph, said element being capable of holding anelectrostatic charge and comprising means for absorbing neutrons saidneutron absorbing means simultaneously emanating secondary radiation,means for enhancing the effect of the neutron beam by capturing saidsecondary emanations and generating a current by dissipation of saidelectrostatic charge in proportion to the number of neutrons absorbed,and means for conducting said current. said neutron absorbing meanscomprising neutron absorbing agents in a concentration of at least l0atoms per cubic centimeter, said neutron absorbing agents having aneutron crosssection of at least one barn, having a ratio of neutronabsorption coefficient to gamma ray absorption eoeffieient of at leastunity. said neutron enhancing means comprising an ionizing radiationsensitive insulative material.

19. An element according to claim 18 wherein the neutron absorbing meanscomprises the element boron.

20. An element according to claim 19 wherein the neutron enhancing meanscomprises an ionizing radiation sensitive insulative material selectedfrom the group consisting of lead oxide and amorphous selenium.

21. A process for using a neutron flux to make a direct image neutronradiograph of a test object on an element, said element being capable ofholding an elec trostatic charge and comprising means for absorbingneutrons and generating a current by dissipation of said electrostaticcharge in proportion to the number of neutrons absorbed, said neutronabsorbing means comprising an insulative layer comprising a neutronabsorbing agent in a concentration of at least l0 atoms per cubiccentimeter,

said process comprising:

l. applying an electrostatic charge to the surface of said element;

2. exposing said test object to said neutron flux in the presence ofsaid element to dissipate the electro' static charge in those areas ofthe surface of said element which correspond to the areas of said testobject which are transparent to said neutron flux, thereby forming anelectrostatic charge image of said test object on the surface of saidelement; and

3. developing said electrostatic charge image.

22. A process using a neutron flux to make a direct image neutronradiograph of a test object on an image recording means, the imagerecording means having means for absorbing neutrons and simultaneouslyproducing secondary emanations. means for accepting and holding anelectrostatic charge. means for insulating the neutron absorbing meansfrom the chargeaccepting means, the insulating means being interposedbetween the neutron absorbing means and the chargeaccepting means andexhibiting increased electrical conductivity when struck by thesecondary emanations, means for conducting charge to or from the chargeaccepting means, and means for conducting charge to or from the neutronabsorbing means,

said process comprising:

l. applying a voltage across the image recording means;

2. exposing the test object to the neutron flux in the presence of theimage recording means. thereby forming an electrostatic charge image ofthe test object on the charge-accepting means of the image recordingmeans; and

3. developing the electrostatic charge image.

1. applying a voltage across the image recording means;
 1. applying anelectrostatic charge to the surface of said element;
 1. An element forreducing a direct image neutron radiograph, said element being capableof holding an electrostatic charge and comprising means for absorbingneutrons and generating a current by dissipation of said electrostaticcharge in proportion to the number of neutrons absorbed, said neutronabsorbing means comprising an insulative layer comprising a neutronabsorbing agent in a concentration of at least 1017 atoms per cubiccentimeter.
 1. AN ELEMENT FOR REDUCING A DIRECT IMAGE NUETRONRADIOGRAPH, SAID ELEMENT BEING CAPABLE OF HOLDING AN ELECTROSTATICCHARGE AND COMPRISING MEANS FOR ABSORBING NEUTRONS AND GENERATING ACURRENT BY DISSIPATION OF SAID ELECTROSTATIC CHARGE IN PROPORTION TO THENUMBER OF NEUTRONS ABSORBED, SAID NEUTRON ABSORBING MEANS COMPRISING ANINSULATIVE LAYER COMPRISING A NEUTRON ABSORBING AGENT IN A CONCENTRATIONOF AT LEAST 10**17 ATOMS PER CUBIC CENTIMETER.
 2. An element forrecording a direct image neutron radiograph, said element being capableof holding an electrostatic charge and comprising means for absorbingneutrons and generating a current by dissipation of said electrostaticcharge in proportion to the number of neutrons absorbed, and meanscontiguous with said neutron absorbing means for conducting saidcurrent, said neutron absorbing means comprising an insulative layercomprising a neutron absorbing agent in a concentration of at least 1017atoms per cubic centimeter.
 2. exposing said test object to said neutronflux in the presence of said element to dissipate the electrostaticcharge in those areas of the surface of said element which correspond tothe areas of said test object which are transparent to said neutronflux, thereby forming an electrostatic charge image of said test objecton the surface of said element; and
 2. exposing the test object to theneutron flux in the presence of the imaGe recording means, therebyforming an electrostatic charge image of the test object on thecharge-accepting means of the image recording means; and
 3. developingthe electrostatic charge image.
 3. developing said electrostatic chargeimage.
 3. An element according to claim 2 wherein the neutron absorbingmeans comprises an insulative layer having a resistivity of at least1010 ohm-cm.
 4. An element according to claim 2 wherein the neutronabsorbing agents have a ratio of neutron absorption coefficient to X-rayabsorption coefficient of at least unity and are present in aconcentration range from about 1021 to about 1023 atoms per cubiccentimeter.
 5. An element for recording a direct image neutronradiograph, said element being capable of holding an electrostaticcharge and comprising means for absorbing neutrons and generating acurrent by dissipation of said electrostatic charge in proportion to thenumber of neutrons absorbed, and means contiguous with said neutronabsorbing means for conducting said current, said neutron absorbingmeans comprising an insulative layer comprising a neutron absorbingagent in a concentration of at least 1017 atoms per cubic centimeter,said neutron absorbing agent having a neutron cross-section per atom ofat least one barn, and having a ratio of neutron mass absorptioncoefficient to gamma ray absorption coefficient of at least unity.
 6. Anelement according to claim 5 wherein the neutron absorbing meanscomprises an insulative layer having a resistivity of at least 1010ohm-cm.
 7. An element according to claim 5 wherein the neutron absorbingagents are present in a concentration range from about 1021 to 1023atoms per cubic centimeter.
 8. An element according to claim 5 whereinthe neutron absorbing agent is an organic material having a highhydrogen content.
 9. An element according to claim 5 wherein the neutronabsorbing agent is an organometallic compound containing an isotope ofan element selected from the group consisting of boron, cadmium,dysprosium, europium, gadolinium, gold, indium, lithium, rhodium,samarium and silver.
 10. An element according to claim 9 wherein theneutron absorbing agent is an organometallic compound containing anisotope selected from the group consisting of boron, 10B, and lithium,6Li.
 11. An element according to claim 9 wherein the neutron absorbingagent is an organometallic compound selected from the group consistingof tri Beta -naphthyl borate, tri Alpha -naphthyl borane, methylamineborane, bis(methylamine)borane, tri(methylamine)borane,gadoliniumhexaantipyrine chloride, hexaantipyrene gadolinium (III)perchlorate, triphenylborine amine, and copper triphenylphosphineborane.12. An element according to claim 9 wherein the organometallic compoundcomprises a tetraarylborate.
 13. An element according to claim 5 whereinthe neutron absorbing agent comprises an element selected from the groupconsisting of berylium, boron, cadmium, carbon, chlorine, eupopium,gadolinium, hydrogen, lithium, nitrogen, oxygen and samurium.
 14. AnElement according to claim 13 wherein the neutron absorbing agent isselected from the group consisting of B2O3, BN, BP, BC, CdS, CdF2, Gd2O3and gadolinium gallium garnet.
 15. An element according to claim 5wherein the neutron absorbing agent is a composition comprising CdS, CdOand B2O3.
 16. An element according to claim 15 wherein the neutronabsorbing agent is a composition comprising about 4.1 percent by weightCdS, about 48.8 percent by weight CdO, and about 47.1 percent by weightB2O3.
 17. An element according to claim 15 wherein the neutron absorbingagent is a composition comprising about 4.9 percent by weight CdS, about46.4 percent by weight CdO, and about 48.7 percent by weight B2O3. 18.An element for recording a direct image neutron radiograph, said elementbeing capable of holding an electrostatic charge and comprising meansfor absorbing neutrons said neutron absorbing means simultaneouslyemanating secondary radiation, means for enhancing the effect of theneutron beam by capturing said secondary emanations and generating acurrent by dissipation of said electrostatic charge in proportion to thenumber of neutrons absorbed, and means for conducting said current, saidneutron absorbing means comprising neutron absorbing agents in aconcentration of at least 1017 atoms per cubic centimeter, said neutronabsorbing agents having a neutron cross-section of at least one barn,having a ratio of neutron absorption coefficient to gamma ray absorptioncoefficient of at least unity, said neutron enhancing means comprisingan ionizing radiation sensitive insulative material.
 19. An elementaccording to claim 18 wherein the neutron absorbing means comprises theelement boron.
 20. An element according to claim 19 wherein the neutronenhancing means comprises an ionizing radiation sensitive insulativematerial selected from the group consisting of lead oxide and amorphousselenium.
 21. A process for using a neutron flux to make a direct imageneutron radiograph of a test object on an element, said element beingcapable of holding an electrostatic charge and comprising means forabsorbing neutrons and generating a current by dissipation of saidelectrostatic charge in proportion to the number of neutrons absorbed,said neutron absorbing means comprising an insulative layer comprising aneutron absorbing agent in a concentration of at least 1017 atoms percubic centimeter, said process comprising: